This proposal is concerned with the consequences of overstimulation on the structure and function of the hair cell stereocilia, and how injury to the sensory hairs impacts on the physiologic behavior of the hair cell itself. The premise guiding this research is that acoustic injury to the inner ear, resulting in lethal or non-lethal damage to the receptor cell, is a direct consequence of abnormal levels of sensory hair bundle stimulation. While the normal mechanisms of hair bundle motion and its relation to hair cell transduction have been extensively studied, the role of hair bundle (stereocilia) damage and its contribution to reduced hair cell function or even destruction is less well explored. The purpose of the present proposal is to reveal the relationship between hair bundle damage and hair cell function. A number of specific hypotheses are proposed to explore the following issues: 1) Can permanent changes in hair bundle motion be produced by specific damage to the extracellular matrix that interconnects the stereocilia in the bundle? 2) Do changes in the relative stiffness of the hair bundle make them more or less susceptible to overstimulation? 3) Can changes in stereocilia stiffness be identified after relatively low levels of overstimulation? 4) What is the effect of reduced hair bundle stiffness following over-stimulation on hair cell function? 5) In the sound damaged chick cochiea, do the surviving tall and short hair cells exhibit normal physiologic behavior immediately after the exposure? These questions are answered through a set of testable hypotheses that are experimentally evaluated by quantitatively analyzing the behavior of hair bundle motion and hair cell electrophysiology. The experiments utilize the expertise gained by the P.I in studying the structural organization of the stereocilia, hair cell micromechanics, and the recovery of function in the sound damaged chick ear. Scanning electron microscopy is used to examine the surface structure of the sensory hairs. In vitro technology is employed to observe and measure sensory hair micromechanics and hair cell electrophysiology. The analysis of relative stereocilia motion is evaluated to determine the role of the extracellular matrix and tip links on hair bundle activity. Modulation of stereocilia stiffness, achieved by varying the level of extra- or intracellular Ca++ concentration, is used to determine if hair bundle stiffness renders them more or less susceptible to overstimulation. Finally, the sound damaged chick ear is used as a model system to evaluate the role of the sensory hairs and hair cells in the process of hearing recovery in this species. These investigations will add to our understanding of the mechanisms that make hair cells susceptible to overstimulation, they will further our appreciation of the damaging and repair process, and may suggest future strageties for protecting the receptor cell, or accelerating its recovery from overstimulation.